OCA Java SE 8 Programmer I

Question 1

If package component is not defined in a class, where does this class resides?

Answer 1

In a default package

Question 2

Can a class have 2 packages defined?

Answer 2

No, it will fail to compile

Question 3

Can import statement be placed before package statement?

Answer 3

No, it will fail to compile

Question 4

How do you structure multi-line comments?

Answer 4

/*
* This
* is
* a comment
*/

Asterisks in every line are not mandatory but they can often be found due to aesthetics reasons.

Question 5

What is the name of java compiler in Windows and Unix systems?

Answer 5

Windows - javac.exe
Unix - javac

Question 6

What is the difference between multi-line comments and Javadoc comments?

Answer 6

Java doc comments start with /**
They are processed with Javadoc which is a JDK tool for generating API documentation of Java classes

Question 7

What are non-access modifiers?

Answer 7

• final
• static
• abstract
• synchronized
• native
• transient

Question 8

Can you use Class instead of class to define a class? :)

Answer 8

No, it is case sensitive

Question 9

What is a class?

Answer 9

Class is a blueprint out of which object is created

Question 10

Can you define multiple classes and/or interfaces in a single Java source file?

Answer 10

Yes, how much you want and in any order you want

Question 11

How many public classes/interfaces can you define in a single Java source code file?

Answer 11

You can define only one public class/interface and name that follows has to match the name of a Java source code file.

Question 12

If you have defined multiple classes with a Java source code file is the import statement propagated to all defined classes?

Answer 12

Yes

Question 13

Can you define more than one executable class?

Answer 13

Yes, you only need to decide which one to call on start-up

Question 14

Define a main metod

Answer 14

public static void main(String args[]) {…}
public static void main(String… args) {…}
static public void main(String args[]) {…}
public static void main(String[] args) {…}

Question 15

Can you define a main method with different signature?

Answer 15

You can, but if you want to run the program you need also a main method with the right signature. Having 2 main methods in the same class like:
- public static void main(String args[]) {…} //will be executed
- public static void main(double number) {…}

is okay since you are only overloading the main method.

Question 16

Write down an example of running a Java class with command line parameters

Answer 16

java MyClass 1 2 3

Question 17

Are package names case sensitive?

Answer 17

Yes

Question 18

How to run a Java class from terminal if you are in different folder than that specific class?

Answer 18

You have to add the base directory to the classpath. Most convenient way would be to do it like:
java -classpath /Work/Projects/java8-certification test.MyClass

Question 19

If you use an import java.lang.* does the size of your class increases?

Answer 19

No

Question 20

If two packages are located in the same level, how do you import a class for another package?

Answer 20

You need to import it using the entire filepath (all packages from base directory)

Question 21

When you use a class defined in java.lang package do you need to import java.lang package?

Answer 21

No, all classes automatically have java.lang package imported

Question 22

Is it possible to use two classes with the same name that are located in different packages (ex. java.util.Data and java.sql.Data) in a particular class?

Answer 22

It is possible, by defining variables like:
java.util.Date date1;
java.sql.Data date2;

Using import statements for both of them is not possible!

Question 23

When you use an import statement like import net.netconomy.* what classes are actually accessable?

Answer 23

Only classes defined directly in net.netconomy folder, classes that are defined in the sub-folder are not imported.

Question 24

How to access classes defined in default package?

Answer 24

Default package classes can only be accessible from the classes that are also in a default package and located in the same directory - classes with named packages are not able to access them.

Question 25

What are static imports?

Answer 25

Static imports are direct imports of public static attributes and methods for another class. package net.netconomy.animal; class Cat { public static int lives = 9; public void sound() { System.out.println("meow!"); } } _________ import static net.netconomy.animal.Cat.*; class Tiger { public void roar() { sound(); } public int livesCounter() { return lives; } } Although they are called static imports, you can only use them as "import static"

Question 26

What is a top-level class?

Answer 26

A top-level class is a class that is not defined within any other class.

Question 27

On which type of Java entities access modifiers can be used?

Answer 27

They can be used on a class/interface and it's members (variables and methods). They can't be used on local variables and method parameters.

Question 28

What access modifiers can we use on a class?

Answer 28

Only public and default (no modifier set). If public is set, class can be accessed outside of the package.

Question 29

What are unrelated classes?

Answer 29

Unrelated classes are classes that don't have inheritance relation.

Question 30

Can related class access protected attributes of inherited classes?

Answer 30

Yes, but it can't access them as reference variables.

Question 31

What is the purpose of non access modifiers?

Answer 31

They are changing the default behavior of a class, interface or method.

Question 32

Can you use abstract modifier on interface?

Answer 32

You can, but it is redundant.

Question 33

Can you use final on a class?

Answer 33

You can use it and that means that the corresponding class cannot be extended.

Question 34

Can you use final on an interface?

Answer 34

No, interfaces are abstract by default, code won't compile.

Question 35

Will this code compile? final StringBuilder name = new StringBuilder("dobar"); name.append("dan");

Answer 35

It will compile, you can't change the value of a final variable directly but you can call a method that will change it's state in the background.

Question 36

Can you use final on a method?

Answer 36

Yes, and that method can't be overriden by inherited class.

Question 37

Can static method access non-static variable?

Answer 37

No, because non-static variable is created when instance is created and static method lives in the class.

Question 38

If a class has static variables and the class is initialized to "null", can you access static variable values?

Answer 38

Yes, they are not connected with class initialization.

Question 39

Can you prefix a class or an interface with "static"?

Answer 39

You can but not for top-level classes, only nested ones.

Question 40

What is the main difference between Java and languages like C or C++?

Answer 40

For C and C++ code has to be recompiled for each system like Windows, Linux, etc. while for Java you can run it via JVM on any system without recompiling.

Question 41

What are the main features and components of Java?

Answer 41

Type safety Object orientation Encapsulation Polymorphism Abstraction Automatic memory management

Question 42

Can a non access modifer "abstract" be used on a class if it doesn't have any abstract methods defined?

Answer 42

Yes

Question 43

What are 8 main primitive types?

Answer 43

char short long int double float boolean byte

Question 44

Define a categorization of primitive types

Answer 44

numerical Boolean signed. | unsigned boolean integers | floating point char byte | int | short | long double | float

Question 45

What is unsigned numeric type?

Answer 45

It can only store positive integer values like "char"

Question 46

Can you define a variable and skip assigning a specific type to it like "price = 12.9;"

Answer 46

No, Java is a typed language

Question 47

What is a literal?

Answer 47

That is a value for a specific type that can be used, like true/false for boolean primitive type.

Question 48

What is the size in bites for each signed primitive type?

Answer 48

byte | 8 bits | -128 to 128 short | 16 bits | -32768 to 32767 int | 32 bits | ~ 2.000.000.000 (+/-) long | 64 bits |

Question 49

What are basic integer literal values?

Answer 49

binary octal decimal hexadecimal

Question 50

How are binary, octal, decimal and hexadecimal values assigned?

Answer 50

binary -> 0b10001 octal -> 0413 decimal -> normal number hexadecimal -> 0x10B

Question 51

Is this a valid code? long number = 100_267_760

Answer 51

Yes

Question 52

Can you use "_" in numbers?

Answer 52

Yes, you can use _ between digits but not on the end or begining. It was designed for grouping digits for different number sytems like decimal, hexadecimal, etc

Question 53

Explain the advantages and disadvantages of float vs double

Answer 53

float is smaller (32 bits vs 64 bits) double is more precise

Question 54

Provide few examples of assigning value to char

Answer 54

char kar = 'A'; char kar = '123'; char kar = '\u0122';

Question 55

How can you assign negative integer value to char?

Answer 55

You can do it by casting -> char kar = (char) -242; The thing is that during conversion "sign" bit is also stored as part of the value which results in strange characters like ﾅ Is see it as distortion of a value

Question 56

Are hyphens allowed in identifiers?

Answer 56

No, only underscore

Question 57

What are valid indentifier names?

Answer 57

- Starts with a-z (big or small) - Can contain digit but not on the first place - Can contain _ on any space - Can contain currency sign on any space

Question 58

When you can't use an underscore in a number?

Answer 58

- before or after . - before letters like F or D in float or double

Question 59

What are the 2 types of variables?

Answer 59

primitive and refernce(object) variables

Question 60

What is a literal value for all object refernces?

Answer 60

null

Question 61

How to assign previously defined integer variables a and b to value 10?

Answer 61

a = b = 10;

Question 62

Can you store larger range of values like long into int?

Answer 62

You can, but only with casting

Question 63

Can you assign multiple variables in the same row?

Answer 63

int a = 5, b = 6, c = 10;

Question 64

What is unary decrement operator?

Answer 64

--a or a++

Question 65

What is the primitive type of a result when adding two primitive values?

Answer 65

Adding two primitive types that are int or smaller from int result always int (ex. byte + short = int) or they result in long if long is one of the numbers

Question 66

How can you make this code compile? short shorty = 123; byte bajti = 100; short rez = shorty + bajti;

Answer 66

final short shorty = 123; final byte bajti = 100; short rez = shorty + bajti;

Question 67

What will this code return? public static void print() { int a = 5; int b = 7; int c = 10; if (a < 3 || b < 10 && c > 20 || b < 9) { System.out.println("YES"); } else { System.out.println("NO"); } }

Answer 67

YES

Question 68

What is autoboxing?

Answer 68

Assigning primitive value to a wrapper class, ex. Double d = 29.3;

Question 69

Will this code compile? Boolean.parseBoolean("TrUe");

Answer 69

Yes, String argument isn't case sensitive.

Question 70

What is the difference when creating wrapper instance using constructor and valueOf() method?

Answer 70

Wrapper classes like Character, Byte, Short, Integer, Long have cached values of -128 to 128. Every time valueOf() method is used with this range of values, cashed values are returned.

Question 71

What will be result of this code? Integer a = Integer.valueOf(10); Integer b = 10; return a == b;

Answer 71

true, autoboxing is also taking cached value

Question 72

Can you compare wrapper classes of different types like Short and Long with ==?

Answer 72

Code won't compile

Question 73

What are different scopes of variables?

Answer 73

- Local scope - Method parameters scope - Instance scope - Class scope

Question 74

How can you call a garbage collection?

Answer 74

You can't really call it, there are methods like System.gc() but there isn't a guarantee that garbage collection will be initiatied.

Question 75

What is a difference between method parameters and arguments?

Answer 75

Arguments ar the actual values transferred to a method.

Question 76

Can you define multiple variable arguments as method params?

Answer 76

No, only one and it has to be last one in the list.

Question 77

Is this code valid? void test(int a,b,c) {}

Answer 77

No

Question 78

What is an initializer block?

Answer 78

It is a block of code { } similar to constructors which is called on creation of an object but before constructors. There are usually used within anonymous classes (type of inner classes without name) since constructors can't exist.

Question 79

Can you call a default constructor (without arguments) if you have a user defined constructor?

Answer 79

No, if you have a user-defined constructor default one is not created.

Question 79

Can you call a default constructor (without arguments) if you have a user defined constructor?

Answer 79

No, if you have a user-defined constructor default one is not created.

Question 80

How to call a constructor from another constructor within the same class?

Answer 80

this("name", 0) It can be called only once and it has to be on the first line.

Question 81

What is the difference when initializing a string value with new String("some value") and with equals sign like = "some value"?

Answer 81

Both of them create a new object in the memory, but if you compare two strings that have been initialized with the same literal value like: String s1 = "some value"; String s2 = "some value"; comparing them like s1 == s2 will return true since they will refer to the same object in the memory. Initializing a String with this value is stored in a pool of Strings.

Question 82

What is the main difference between StringBuilder and StringBuffer?

Answer 82

Both of them represent mutable implementation of String (immutable) but StringBuffer is providing synchronized support for multiple threads. Always use StringBuilder when you have only one thread running since it is faster.

Question 83

How String ensures immutability?

Answer 83

When a new String is initialized, literal value is stored in a private final array of characters. Immutability is ensured since it is an array (size cannot change after initialization) and it has a final non-access modifier

Question 84

If String is an immutable object how come methods like replace() are replacing its value?

Answer 84

They are not replacing value of that String, they return a completely new String.

Question 85

What method does String have?

Answer 85

Query positions of chars: - indexOf() - chartAt() Seem to modify String: - substring() - trim() - replace() Other: - length() - startsWith() - endsWith()